1. Field of the Invention
The present invention relates to processing a image signal of a TV, and in particular to an improved apparatus for simultaneously displaying TV and PC (personal computer) images on a TV display by dividing the TV and PC images.
2. Description of the Conventional Art
FIG. 1 is a circuit diagram of a TV system having a conventional double screen display function. As shown therein, the circuit includes a microcomputer 106 for outputting a first control signal cs and a second control signal dw in order to display a double screen according to a viewer's selection; an audio/video switch 101 for outputting a single signal among a main image signal Vim which passes through a main tuner and an intermediate frequency processing unit (not illustrated) in accordance with the first control signal cs, a sub image signal Vis which passes through a sub tuner and the intermediate frequency processing unit, and an external input signal Vex; a first comb filter 102 for separating the main image signal Vim which is outputted from the audio/video switch 101 into a main luminance signal Ym and a main chrominance signal Cm by using a subcarrier frequency f.sub.sc of 3.58 MHz and applying the two signals to the audio/video switch 101; a second comb filter 103 for separating the sub image signal Vis or the external input signal Vex which is received from the audio/video switch 101 into a sub luminance signal Ys and a sub chrominance signal Cs and then outputting the two signals; a double screen control unit 104 for receiving the main luminance signal Ym and the main chrominance signal Cm which are received from the audio/video switch 101 and the sub luminance signal Ys and the sub chrominance signal Cs which are received from the second comb filter 103, and for outputting a double screen luminance signal Ydw and a double screen chrominance signal Cdw; a double screen switch 105 for outputting the main luminance signal Ym and the main chrominance signal Cm which are received from the audio/video switch 101 in accordance with the second control signal dw of the microcomputer 106 or outputting the double screen luminance signal Ydw and the double screen chrominance signal Cdw which are received from the double screen control unit 104; a video chroma signal processing unit 107 for converting the luminance signal Y and the chrominance signal C which are received from the double screen switch 105 to primary color signals R, G, B, and amplifying and outputting the primary color signals; and a RGB amplifier 108 for converting the amplified R, G, B signals to a magnitude which is needed for driving a picture tube 109.
Here, the external input signal Vex is a signal which is inputted to an external input terminal (not illustrated) of the TV, such as a video image signal.
The operation of the conventional TV system will now be described in detail.
When the main image signal Vim which passed through the main screen tuner and the intermediate frequency processing unit (not illustrated), the sub image signal Vis which passes through the sub screen tuner and the intermediate frequency processing unit, and the external input signal Vex are inputted to the audio/video switch 101, the audio/video switch 101 sends the main image signal Vim to the first comb filter 102 through a first output terminal Vout1.
On the other hand, a viewer who decides to watch a picture (an image) according to the sub image signal Vis or the external input signal Vex controls the microcomputer 106, and the sub image signal Vis or the external input signal Vex is inputted to the second comb filter 103 through a second output terminal Vout2 in accordance with a logical state of the first control signal cs, which is changed by to the control of the viewer. The first comb filter 102 separates the main image signal Vim into the main luminance signal Ym and the main chrominance signal Cm by using the subcarrier frequency f.sub.sc of 3.58 MHz, and the main luminance signal Ym and the main chrominance signal Cm pass through the audio/video switch 101, and then are inputted to the double screen control unit 104 and double screen switch 105.
The second comb filter 103 separates the sub image signal Vis or the external input signal Vex received from the second output terminal Vout2 into the sub luminance signal Ys and the sub chrominance signal Cs and sends the two signals to the double screen control unit 104.
The double screen control unit 104 converts each inputted signals Ym, Cm, Ys, and Cs into a digital signal, reads and writes each signal in a memory, and again converts the digital signal to an analog signal. The double screen control unit 104 generates the double screen luminance signal Ydw and double screen chrominance signal Cdw by having a different read and write timing of each memory, and sends the two signals to the double screen switch 105, thereby performing a double window or PIP. Here an applied frequency band is approximately 2 MHz.
The viewer decides whether to watch a single or double screen and controls the microcomputer 106, and accordingly a logical state of the second control signal dw of the microcomputer 106 is changed, and the double screen switch 105 selects each signal from the above signals Ym, Cm, Ydw, and Cdw in accordance with the logical state of the second control signal dw and outputs the selected signals to the video chroma signal process unit 107. The inputted signals Y and C are converted to the primary color signals R, G, B by an inner matrix in the video chroma signal process unit 107 and amplified up to the magnitude for which the RGB amplifier 108 needs to drive the picture tube 109, and applied to each RGB cathode of the picture tube 109, whereby a single screen appears according to the main image signal, or a double screen appears according to the main and sub image signals, or the main image and external input signals.
Therefore, the viewer can watch the single or double screen by controlling the microcomputer 106.
In addition, the viewer also can watch a double screen processed in a personal computer PC by using the conventional art as described above. In this case, when the viewer connects the output terminal for outputting a image signal of the PC to an external input terminal, the image signal processed in the PC is inputted to the audio/video switch 101 through the external input terminal. That is, the external input signal Vex of the audio/video switch 101 becomes a PC image signal.
Also, when the viewer controls the microcomputer 106, the double screen switch 105 transits the double screen luminance signal Ydw and the double screen chrominance signal Cdw outputted from the double screen control unit 104, thus the double screen according to the main image signal and the PC image signal can be displayed. Accordingly, the viewer is able to watch the TV and PC image simultaneously.
The frequency band of the TV image signal is approximately 2 to 3 MHz, and the frequency band of the PC image signal with a high resolution is approximately 20 MHz in a VGA mode. Namely, a horizontal frequency of the PC image signal is about 31.5 kHz, twice as much as a TV horizontal frequency, and if the number of horizontal dot is 640, the frequency band is 31.5 KHz*640, that is, about 20 MHz.
In order to display the PC image of a high resolution, a TV system should be able to process the frequency band of 20 MHz. However, in the conventional art, when the PC image signal is inputted to the audio/video switch 101, the PC image signal passes through the double screen control unit 104, here the frequency band of about 2 MHz is applied for the double screen control unit 104, thus the resolution of the PC image is considerably dropped because it needs the frequency band about 20 MHz.